The present invention pertains to the field of polymer chemistry and specialty chemicals. In particular, the present invention pertains to the field of specialized chemicals that are often associated with dyeing nylon fibers, among other uses. More specifically, the invention pertains to metal salts of a dialkyl ester of 5-sulfoisophthalic acid and methods of preparing them.
The term “nylon” generally encompasses a family of synthetic polymers first developed by DuPont in the 1930's. Since then nylon and nylon fibers have found use in products as diverse as carpet fibers, clothing, sutures, parachutes, and footwear, among other uses.
Nylon fibers, especially those used for carpet fiber, are often classified as to type, depending on the fiber's receptivity to acid dyes and basic or cationic dyes. Cationic dyeable nylon fibers generally exhibit inherent stain resistant properties as compared to other nylon types but traditionally suffered from poorer lightfastness, especially in light shades. This resulted in the under-utilization of cationic dyeable nylon as a carpet fiber.
As expected, considerable time, energy, and resources were devoted to finding new and improved methods to improve the dye absorbing characteristics of cationic dyeable nylon. Over the years, several methods were developed in which very specialized chemicals were added to the basic nylon dyeing process to impart improved cationic dye-ability to the polymer. Metal salts of dialkyl esters of 5-sulfoisophthalic acid are one family of such chemicals. The sodium salt of dimethyl-5-sulfoisophthalic acid (aka: DMSSIPA) is a well known. A few other metal salts are known (such as the lithium salt) and can be used as polymer additives. Research into these salts continues to further refine their polymer modifying characteristics and explore additional uses.
The term “drowning” as used herein means the addition of one liquid component to another liquid component. In other words, the term means pouring a solution or intermediate slurry into a second liquid. As used herein, the term usually refers to the addition of an intermediate solution containing a dialkyl ester of 5-sulfoisophthalic acid to a buffered aqueous solution.
The preparation of any salt of a dialkyl ester of 5-sulfoisophthalic acid typically contains a step in which the dialkyl ester is contacted with a metal cation to form the metal salt.
The contacting step is often accomplished by drowning a solution containing a dialkyl ester of HSIPA into a neutralizing solution buffered by a base such as NaOH, Na2CO3, NaHCO3, or Na2SO4 to form the salt product. A further alternative is to add the HSIPA to a single solution containing an alcohol and a base.
The resulting product, the metal salt of a dialkyl ester of 5-sulfoisophthalic acid, is then separated, usually by some filtration method, purified and packaged.
There are several problems associated with these traditional methods of forming metal salts of dialkyl esters of HSIPA. Use of sulfate bases to buffer the drowning step is undesirable because sulfate precipitates on nylon filaments, which leads to filament breakage, decreased product throughput, and inferior product.
In addition, a hazardous byproduct (methyl hydrogen sulfate) is formed in substantial amounts in some processes that utilize high levels of sulfate. Extra equipment and record keeping is required to appropriately handle this hazardous byproduct, which increases production costs.
Similarly, problems exist in those traditional processes that utilize carbonates to buffer the system. The dialkyl ester of HSIPA reacts with the carbonate to produce CO2 gas (a process known as “degassing”), which then generates foam in the product slurry, which in turn complicates materials handling and increases costs. The CO2 may also be subject to regulatory reporting/control, which increases costs.
Accordingly, there is a need for a new method of producing metal salts of the dialkyl ester of HSIPA that avoids the problems associated with current production methods. In particular, there is a need for a manufacturing process that reduces the use of sulfates to reduce or substantially eliminate sulfate related filament breakage. There is also a need for a manufacturing process that reduces the formation of methyl hydrogen sulfate and the regulatory issues associated with it. There is a need for a manufacturing process that reduces or substantially eliminates the costs and problems associated with carbonate based degassing. Finally, there is a need for a method that can serve as a platform to make multiple metal salts of dialkyl esters of 5-sulfoisophthalic acid.
In addition, there is a need for new metal salts of dialkyl esters of 5-sulfoisophthalic acid to expand the portfolio of salts that are available to polymer chemists.
If one looks at the general needs discussed above, it becomes apparent that a common factor related to each is the manner in which the reaction system is buffered. Metal sulfates as the source of metal cations do not provide the necessary buffering and adds undesirable sulfates which can lead to inferior product. Carbonate based buffers lead to degassing, which complicates manufacture. Thus, although there is a general need for a new manufacturing process, a more precise recitation of the industry's need would be: there is a need for a new buffering system for use in making metal salts of dialkyl esters of 5-sulfoisophthalic acid.
The process according to the invention provides a new buffering system and provides a high quality product in a manufacturing process that is efficient from both an engineering and economic perspective. The invention also serves as a platform technology that can be used to make multiple metal salts of dialkyl esters of 5-sulfoisophthalic acid, including several novel salts. Additional benefits of the invention are discussed at the end of the detailed description so that they can be understood in their technical context.